1. Field of the Invention
The present invention relates to an appropriate table support apparatus for a wafer stage with a projection exposure apparatus or an EB (electron beam) exposure apparatus for production of semiconductor elements.
2. Description of the Related Art
In the projection exposure apparatus or the EB (electron beam) exposure apparatus for production of semiconductor elements, a wafer is automatically transferred to a sample stand and fixed to a wafer holder. It is apparently flat, but is actually subject to a slight warp or distortion from processes such as a high-temperature process, etc. At some points, an exposure plane is not vertical to the optical axis of a projection lens. Therefore, a sample stand is equipped with a chip levelling mechanism for amending the tilt of the exposure plane of the wafer in relation to an image plane of the projection lens.
FIG. 1 shows an example of a sample table equipped with a chip levelling mechanism. A sample table 1 includes a base 2 fixed to an XY stage; a plurality of vertically flexible and, for example, piezoelectric drives device 3 mounted on the base 2; a holder table 4 supported by each drive device 3; a wafer holder 5, provided on the holder table 4, for holding a wafer W by adsorption; a leaf spring 6, horizontally mounted between the base 2 and the holder table 4, for locking the holder table 4 in a horizontal state; a moving mirror 7; a laser interferometer 8; and a sensor 9. With the above described configuration, the exposure plane of the wafer W is amended in relation to the image plane of the projection lens by moving the holder table 4 up and down or tilting the holder table 4 in an optional direction by appropriately adjusting the amount of drive of each drive device 3.
The sample table 1 further includes a damper mechanism 10 for damping the vibration generated at the holder table 4 when the level of the exposure plane is adjusted. The damper mechanism 10 is provided between the base 2 and the holder table 4, and damps the vibration of the holder table 4 based on the viscosity of a viscoelastic substance. Furthermore, the position of the holder table 4 in the X and Y directions is measured by the laser interferometer 8.
Recently, a larger wafer is designed, and a sample stand is designed in a way that a holder table can be responsive and better in movement when the level of the exposure plane is adjusted. However, in the sample stand having the above described damper mechanism, the holder table is strongly held in the opposite direction of its movement with the resistance of a viscoelastic substance when a level adjustment is made, thereby having the holder table deformed. Since the resistance to the holder comes from the viscosity of the viscoelastic substance, the holder table can recover from the deformation with time. However, the exposure plane cannot be correctly aligned until the holder table can be free of deformation. This is a problem in improving the responsiveness of the holder table.